With an increase in energy demand in recent years, the demand has been increasing for a gas turbine for mechanical drive which is suited for the production of liquid natural gas (LNG). LNG plants accomplish liquefaction by pressurizing the natural gas by means of an LNG liquefying compressor. The two-shaft gas turbine in particular is commonly used for driving the LNG liquefying compressor.
In the two-shaft gas turbine, a turbine part is divided into a low pressure turbine and a high pressure turbine. The low pressure turbine is responsible for driving an LNG compressor or load while the high pressure turbine, as the gas generator, is connected with the compressor. The two-shaft gas turbine has a feature that each of the high pressure turbine and the low pressure turbine has a rotation shaft independent from each other.
The two-shaft gas turbine is used not only for the above-described mechanical drive but may also be used for power generation to be connected to an electric generator. Because of its simple structure and easy operation, a one-shaft gas turbine featuring coaxial rotation of the compressor and the turbine is predominantly used as a gas turbine for power generation. However, in the case of downsizing, the one-shaft gas turbine suffers from the disadvantage of requiring a decelerator because the gas turbine needs to maintain the rotational speed in accordance with the specifications of the electric generator.
On the other hand, the use of the two-shaft gas turbine for power generation negates the need for the decelerator because a rotational speed of the gas generator including the compressor, combustor and high pressure turbine and a rotational speed of the low pressure turbine can be chosen freely. Accordingly, a compact and highly efficient gas turbine can be provided.
In the operation of such a two-shaft gas turbine, the set angle of an inlet guide vane (hereinafter, referred to as IGV) of the compressor is regulated based on a corrected speed of a gas generator shaft, the corrected speed obtained by considering the influence of the ambient temperature on an actual speed of the gas generator shaft. It has been a common practice to provide an IGV control based on the corrected speed regardless of an operating state of the gas generator.
In this case, the IGV set angle varies according to the corrected speed correlated with the ambient temperature as illustrated in FIG. 9A showing a relation between the corrected speed of the gas generator shaft and the IGV set angle. (The relation between the corrected speed of the gas generator shaft and the IGV set angle is uniquely decided.) Therefore, the operation line varies as illustrated in FIG. 9B showing a relation between the actual speed of the gas generator shaft and the IGV set angle. As a result, the rotational speed of the gas generator shaft varies depending upon the ambient temperature. That is, even when the gas turbine is operated in the vicinity of the rated load by increasing the IGV set angle, the actual speed of the gas generator shaft varies depending upon the ambient temperature.
This leads to the increase in a region of blade resonance avoidance during the rated load operation, which makes resonance avoidance design more difficult. Further, the increase in the region of resonance avoidance means a decrease in the freedom of airfoil design. This makes it more difficult to improve the aerodynamic performance of the airfoil.
With an aim to avoid the above-described resonance during the rated load operation, a control method of the two-shaft gas turbine is disclosed in JP 2011-38531. JP 2011-38531 focuses attention on a fact that the importance of avoiding surge (fluid pulsation phenomenon induced by flow separation from compressor blades) by way of the control based on the corrected speed decreases during the high speed rotation including the rated load operation condition. The two-shaft gas turbine including the gas generator and the low pressure turbine adopts an IGV control measure in which the IGV set angle is controlled based on the corrected speed of the gas generator shaft during the low speed rotation thereof and the IGV set angle is controlled to maintain a constant actual speed of the gas generator shaft during the high speed rotation thereof.
As illustrated in FIG. 10A showing a relation between the corrected speed of the gas generator shaft and the IGV set angle and illustrated in FIG. 10B showing a relation between the actual speed of the gas generator shaft and the IGV set angle, the use of the control method of the two-shaft gas turbine disclosed in JP 2011-38531 provides operations that the operation line under a low load is constant regardless of the ambient temperature while the corrected speed of the gas generator shaft under a high load varies depending upon the ambient temperature. On the other hand, the operation line under a low load varies depending upon the ambient temperature while the rotational speed of the gas generator shaft under a high load is constant.
Therefore, the control method can effectively eliminate the resonance problem (the problem that the resonance is induced by the rotation speed of the fast-rotating gas generator shaft approaching a resonant rotation speed, increasing a risk of damaging turbine rotor or compressor rotor). Further, the control method can effectively cope with the compressor surging during the low speed rotation. Thus, the control method decreases burden on design related to the resonance problem and facilitates the above-described resonance avoidance design. Furthermore, an improved aerodynamic performance due to the increased freedom of airfoil design is prospected.
As described above, when the control method of two-shaft gas turbine disclosed in JP 2011-38531 is used, both the surge particularly encountered during the low speed rotation including startup and shut-off and the resonance encountered during the high speed rotation including the rated load operation can be avoided.
However, the surge can possibly occur during the high speed rotation although the importance for avoidance is low. Therefore, depending on the operation condition, the control method of the two-shaft gas turbine disclosed in JP 2011-38531 may fail to keep enough surge margin. Particularly under a high ambient temperature condition (30° C. or more, for example) where the actual speed is constant, a margin for the surge normally decreases and hence, the problem of the surge avoidance is likely to emerge.
In view of the above, the present invention aims to provide a two-shaft gas turbine which includes the gas generator and the low pressure turbine and which can ensure the surge avoidance during the high speed rotation under the high ambient temperature condition as well as the above-described surge avoidance during the low speed rotation and the resonance avoidance during the high speed-rotation, and to provide a control system and control method thereof.